It is known in the industry that reciprocating pistons for internal combustion engines are subject to lateral play or side thrusts due to the necessary clearance between the piston and the surrounding cylinder wall or liner and the angularity of the piston rod. Impact between the piston and the cylinder wall or liner, due to side thrusts, is commonly known as piston slap which provides objectionable noise to an engine. Piston slap is most severe at cold operating temperatures wherein the maximum clearance between the piston and the cylinder wall or liner is realized. As the engine temperature increases and the piston expands, the clearance is reduced and the severity of piston slap is reduced. However, due to the difficulty of controlling the dimensions of the piston and the cylinder wall to maintain exactly optimum (or minimum) clearance at the maximum temperature, a small amount of clearance must be designed into the assembly. Thus, piston slap can be prominent at high operating temperatures as well. In addition to the noise created by the slapping of the piston against the cylinder wall or liner, a second problem is that cavitation can occur on the outside surface of the cylinder wall or liner. Cavitation occurs when the cylinder wall or liner vibrates due to the impact of the piston against it, thereby creating compression waves in the coolant. Each compression wave is followed by a rarefaction wave. The result is positive to negative pressure reversals in the liquid which can occur from a few hundred to several thousand times per second depending on the condition of the engine. These pressure reversals cause the formation and subsequent collapse of cavities in the liquid. During the negative portion of the pressure oscillation, gas and vapor filled cavities in the colant nucleate and grow. When the growing cavity is subjected to the high pressure portion of the cycle, its growth is arrested or reversed. The cavities then either oscillate or implode. Those cavities which implode release large localized hydraulic pressures onto the outside surface of the cylinder wall and cause a pitting action.
Various prior art patents which sought to overcome the piston slap problem are U.S. Pat. Nos. 2,208,782 issued to J. Workman on July 23, 1940; 2,049,922 issued to G. C. Nienow on Aug. 4, 1936; 2,966,382 issued to E. R. Olsen on Dec. 27, 1960; 3,115,070 issued to Chi M. Tsang on Dec. 24, 1963 and 4,158,328 issued to Beardmore on June 19, 1979. These patents all use some type of an insert or positioning means inserted into the body of the piston to maintain a portion of the piston in engagement with the cylinder wall or liner. Although the teaching of these patents may alleviate the piston slap problem to a certain degree, there is still a desire to come up with an improved method. In addition, future internal combustion engines will most likely make use of insulated pistons having a greater mass and weight than current pistons. The increased mass and weight will result in even higher piston impact forces and therefore piston slap will be more prominent and detrimental to the engine.
Now a reduced impact piston assembly has been invented which will minimize the piston slap problem in both current and a future engine cylinders.